Initially, offshore well operations were conducted from fixed platforms in relatively shallow water. As such operations moved from shallow to deeper water, the cost of fixed platforms became very expensive. In attempts to reduce expense and to conduct well operations in deep water, different systems have been proposed, such as providing a completely submerged production station at the sea floor, providing a floating platform production system in which semisubmersible platforms are utilized, and more recently, providing a tension leg platform production system. In the floating platform production system, a subsea manifold at the sea floor may receive production from satellite subsea wells where individual production risers transmit production flow to the platform where it is processed and then returned to the sea floor manifold for delivery to shore facilities, for temporary storage, or delivered to a tanker. Production and transfer risers in such a system are supported by riser tensioners which accommodate heave of the platform in the order of ten feet.
Tension leg production system involves the use of tension leg platforms, such as described in U.S. Pat. Nos. 3,780,685, 3,648,638, and 3,154,039. Tension leg platforms are characterized by the absence of heave, roll or pitch in response to wave motion and thus provides opportunity for improved production efficiency and simplification of the riser design and tensioning thereof.
In prior proposed offshore well installations, template means for defining the location of a group of wells have been employed. Such template means have been constructed with permanent guide posts associated with each well to facilitate location of the well and to provide means for attaching guide lines to the guide posts for lowering well tools and equipment for maintaining or servicing each well. Such multiwell template means sometimes provided for installation of a wellhead and production modules on the template means which included piping and control lines which required makeup connections at the template. Substantially precise alignment of the pipe and control lines was necessary to make a connection, and this was often difficult in deep water and required the use of divers, diving capsules, mechanical manipulators and robots under remote control and special equipment.
In such prior proposed offshore well operations, riser systems extended from a wellhead at the sea floor to a floating vessel; and as mentioned above, riser tensioning means were required as well as heave compensating means. When such riser systems included any great length in deep water, ball and socket joints were often used at the wellhead to permit lateral movement of the riser caused by water currents and motion of the floating platform. Various systems were used to support the riser to accommodate such motion and change in relative position of the riser and platform.